This invention relates generally to a process for the recovery of oil from subterranean formations, and more particularly to a gas flooding process.
Petroleum or oil is generally recovered from subterranean formations by penetrating the formation with one or more wells and pumping or permitting the petroleum to flow to the surface through the well. In various recovery operations, an external driving force is not required to drive the petroleum to the producing well and/or the surface. For example, some natural driving energy such as an underlying active water drive or a gas under some minimum pressure may possess sufficient pressure to drive the petroleum or hydrocarbon to the well and then to the surface. Recovery of petroleum using natural energy is referred to as primary recovery.
In many instances, the natural driving energy is insufficient or becomes insufficient to cause the petroleum to flow to the well. For example, a substantial portion of the petroleum to be recovered may remain in the formation after depletion of the natural driving energy. In other cases, the subterranean formation, while containing substantial amounts of petroleum, may not possess the necessary driving force to recover any of the petroleum. In such cases, various techniques have been applied heretofore to recover the petroleum. Although such techniques are commonly referred to as secondary recovery, in fact, they may be primary, secondary or tertiary in sequence of employment.
One conventional method for the secondary recovery of petroleum from a subterranean formation involves injecting water through one or more injection wells to drive the residual petroleum or oil towards a producing well. However, water alone does not efficiently displace petroleum. Therefore, it has become a common practice to add a variety of materials to the drive water to improve the efficiency of the flooding operation. Specifically, it is a common practice to add a surfactant such as a petroleum sulfonate to the drive water. The surfactant reduces the interfacial tensions between the water and the oil, thereby making the oil more miscible with the water and increasing oil recovery. In general, the lower the interfacial tension between the oil and water, the better the performance of the water flooding operation.
In various operations, the water or water/surfactant mixture channels through the formation such that a disproportionately high amount of the water passes through zones of high permeability into the producing wellbore without contacting appreciable amounts of oil in the reservoir, particularly that oil contained in zones of low permeability. This greatly reduces the efficiency of the operation. There are a number of methods to control the flow of drive water through the subterranean formation.
An alternative method of secondary oil recovery involves using steam, particularly in secondary oil recovery methods for heavy oils. The steam reduces the viscosity and, hence, increases the flowability of the oil. One such method is cyclic steam stimulation (so-called "huff-n-puff" method for the secondary recovery of oil) wherein, in one portion of the cycle, steam is injected into a producing well and, in a second portion of the cycle, oil is recovered from the producing well. A second method is a steam flooding operation wherein steam is injected into an injection well to drive the oil to the producing well. To improve the efficiency of these recovery methods, steam injection is often alternated with injections of a surfactant solution which is capable of foaming. The resulting foam controls the mobility of the following steam as it passes through the formation by rendering it more difficult for the steam to flow through the paths previously swept by the steam. In general, the ability of the surfactant to reduce the interfacial tension between the steam and the oil is not as important as the ability of the surfactant solution to form a stable foam and may be of only minor, if any, importance. Therefore, surfactants which are useful in water flooding are not necessarily useful, and are often not useful, in the secondary oil recovery methods using steam. The surfactant employed in secondary oil recovery methods involving steam is exposed to high temperatures (e.g., 175.degree. C. to 232.degree. C.) and water of relatively high purity (i.e., water having a low dissolved solids content) and are chosen accordingly.
Yet another method of secondary oil recovery is gas flooding which involves injecting a gas such as carbon dioxide or nitrogen into the formation through one or more injection wells to drive the oil in the reservoir towards a producing well. In a gas flooding operation, the gas can be injected as a solution or dispersion with water. Alternatively, the gas can be injected without water and, in such case, will often form either a solution or dispersion with water which naturally exists in the formation or which has been injected either previous or subsequent to the gas injection. Although gas or a gas/water mixture can be employed alone, in general, gas flooding comprises alternatively injecting gas and drive water. In theory, the gas or gas/water mixture thins or solubilizes the oil and the drive water pushes the gas or gas/water mixture and oil to a producing well.
Unfortunately, the gas or gas/water mixture is prone to channel through the formation such that a disproportionately high amount of the gas bypasses through zones of high permeability into the producing wellbore without contacting appreciable amounts of oil in the reservoir. To prevent channeling of the gas and drive water and to otherwise control the mobility of the drive fluids, thereby increasing oil production, it has been suggested to employ a foam prepared from a mixture of water and a surfactant during the gas flooding operation. Such mixture has been found to prevent channeling and to force the drive fluids into the less permeable zones, thereby increasing oil production.
Surfactants which have been found to be useful as a means of modifying the profile in a gas flooding operation are surfactants capable of forming a foam with an aqueous liquid and include alkyl polyethylene oxide sulfates (see, for example U.S. Pat. No.4,113,011); polyalkoxy sulfonates (see, for example U.S. Pat. No. 4,502,538); polyalkoxylated alcoholic or phenolic surfactants (see, for example U.S. Pat. No. 4,380,266) and the like. The surfactant employed in a gas flooding operation is exposed to relatively low temperatures (e.g., less than 95.degree. C.) and water of relatively low purity (i.e., water having a relatively high dissolved solids content). Therefore, surfactants which may be useful in water and/or steam flooding may not necessarily be useful, and are often not useful, in the secondary oil recovery methods using gas flooding techniques.
However, the surfactants heretofore taught to be effective in modifying the mobility of the drive fluids in gas flooding operations have not proved to be particularly effective in every operation. Specifically, in certain instances, greater foam stabilities are desired to achieve the desired increase in oil production. In other instances, the surfactants have not been found to be particularly effective regardless of their foaming ability as measured in the laboratory.
In view of the deficiencies of the prior art methods for improving the mobility of the drive fluids in a gas flooding operation, it remains highly desirable to provide an improved method for controlling the mobility of the drive fluids in a gas flooding operation.